Measuring the Mean Free Path of H i Ionizing Photons at 3.2 ≤ z ≤ 4.6 with DESI Y1 Quasars

Journal Article · Wed Mar 05 00:00:00 EST 2025 · The Astrophysical Journal. Letters

DOI:https://doi.org/10.3847/2041-8213/adb48f· OSTI ID:3007174

Gao, Anning; Prochaska, J. Xavier; Cai, Zheng; Zou, Siwei; Zhao, Cheng; Sun, Zechang; Ahlen, S.; Bianchi, D.; Brooks, D.; Claybaugh, T.; de la Macorra, A.; Dey, Arjun; Doel, P.; Forero-Romero, J. E.; Gaztañaga, E.; Gontcho A Gontcho, S.; Gutierrez, G.; Honscheid, K.; Juneau, S.; Kremin, A. ; Martini, P.; Meisner, A.; Miquel, R.; Moustakas, J.; Muñoz-Gutiérrez, A.; Newman, J. A.; Pérez-Ràfols, I.; Rossi, G.; Sanchez, E.; Schubnell, M.; Sprayberry, D.; Tarlé, G.; Weaver, B. A.; Zou, H.

The mean free path of ionizing photons in the intergalactic medium (IGM; λ $$^{912}_{mpf}$$ ) is a crucial quantity in modeling the ionization state of the IGM and the extragalactic ultraviolet background, and it is widely used in hydrodynamical simulations of galaxies and reionization. We construct the largest quasar spectrum data set to date—12,595 signal-to-noise ratio > 3 spectra—using the Y1 observations of the Dark Energy Spectroscopic Instrument, to make the most precise model-independent measurement of the mean free path at 3.2 ≤ z ≤ 4.6. By stacking the spectra in 17 redshift bins and modeling the Lyman-continuum profile, we get a redshift evolution λ $$^{912}_{mpf}$$ ∝ ( 1 + z ) ^{− 4.27} at 2 ≤ z ≤ 5, which is much shallower than the previous estimate, λ $$^{912}_{mpf}$$ ∝ ( 1 + z ) ^{− 5.4} . We then explore the sources of systematic bias, including the choice of the intrinsic quasar continuum, the consideration of Lyman-series opacity and Lyman-limit opacity evolution, and the definition of λ $$^{912}_{mpf}$$ . Combining our results with estimates of λ $$^{912}_{mpf}$$ at higher redshifts, we conclude at high confidence that the evolution in λ $$^{912}_{mpf}$$ steepens at z ≈ 5. We interpret this inflection as the transition from the end of H i reionization to a fully ionized plasma that characterizes the IGM of the past ∼10 billion yr.

• Research Organization: Andes U., Bogota; Barcelona, IEEC; Barcelona, IFAE; Barcelona, Polytechnic U.; Beijing Observ.; Boston U.; Chicago U., KICP; Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States); Hiroshima Shudo U.; ICREA, Barcelona; KEK, Tsukuba; Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Lick Observ.; Madrid, CIEMAT; Michigan U.; Milan U.; Natl. Astron. Observ. of Japan; Natl. Solar Observ., Tucson; Ohio State U.; Ohio State U., Dept. Astron.; Pittsburgh U.; Portsmouth U., ICG; Sejong U.; Siena Coll., Loudonville; Tokyo U., IPMU; Tsinghua U., Beijing; U. Michigan, Ann Arbor; UNAM, Mexico; Univ. of Michigan, Ann Arbor, MI (United States); University Coll. London
• Sponsoring Organization: National Science Foundation; US Department of Energy; USDOE Office of Science (SC), High Energy Physics (HEP)
• Grant/Contract Number: 89243024CSC000002; AC02-05CH11231; SC0019193
• OSTI ID: 3007174
• Alternate ID(s): OSTI ID: 2479774
• Report Number(s): FERMILAB-PUB-24-0863-PPD
• Journal Information: The Astrophysical Journal. Letters, Journal Name: The Astrophysical Journal. Letters Journal Issue: 2 Vol. 981; ISSN 2041-8205; ISSN 2041-8213
• Publisher: IOP Publishing
• Country of Publication: United States
• Language: English

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79 ASTRONOMY AND ASTROPHYSICS